Can I use an 8 amp diode to stop alt leaking back on switch?

Shealray · Nov 21, 2013

I got on the internet and found how to read diodes. I have a diode with one stripe gray meaning it is an 8 amp diode. I was told to use a 5 amp diode or (2) 3 amp diodes wired parallel. I am thinking an 8 amp would work also. Is this correct thinking?

Thanks for the help

JMOR · Nov 21, 2013

(quoted from post at 17:02:10 11/21/13) I got on the internet and found how to read diodes. I have a diode with one stripe gray meaning it is an 8 amp diode. I was told to use a 5 amp diode or (2) 3 amp diodes wired parallel. I am thinking an 8 amp would work also. Is this correct thinking?

Thanks for the help

Amps & up......all the way to 1,000 of you like and can afford it!

travis miranowski · Nov 21, 2013

Instead of wasting time trying to rig in a diode, why not just go to local auto parts store and get a alternator lead with the diode inline already???? less than $10.00 and a lot less messing around!

davido30093 · Nov 21, 2013

" I have a diode with one stripe gray meaning it is an 8 amp diode"

WOW! That"s about as unreal a comment as I have ever read. You don"t need an 8 amp diode, or even a 5 amp diode and the grey stripe has NOTHING to do with it"s current rating.

Try a search. There is plenty of information already posted.

John T · Nov 21, 2013

If you"re talking about a device to prevent the Delco 10SI family of three wire alternators from backfeeding the ignition coil when you turn the IGN Off,,,,,,,,,YES that will work fine. Ive seen much smaller ones then that still work and of course, higher rated amps will also (though not needed)

Other devices that can work are a 10 ohm resistor (also provides some degree of current limiting protection),,,,,,,,,,,,,A small Incandescent Idiot Light (also works as an idiot charge indicator light),,,,,,,,,,,,,,An idiot light in parallel with a resistor so it still works even if the lights burned out.

John T

Mike (WA) · Nov 21, 2013

So what does the grey stripe signify?

Royse · Nov 21, 2013

(quoted from post at 18:15:29 11/21/13) So what does the grey stripe signify?

Stripe indicates the cathode end of the diode.

rvirgil_KS · Nov 21, 2013

A diode is a device that passes current in one direction and blocks current in the opposite direction. It is used with a Delco alternator to pass start up current but block reverse current that would allow an engine to continue running after the switch is turned off.

A diode has two ratings, forward current and reverse voltage. For instance a diode rated 3 Amps @ 50 volts means it is rated to conduct up to 3 amps in forward direction or withstand up to 50 volts in reverse direction without failure.

Delco start up current is less than half an amp. I have done a number of alternator conversions and have done several with 1.5A @ 50 volts and experienced no diode failures. Higher amp or volt ratings just give extra reserve protection.

The marked band signifies the cathode or negative end and has nothing to do with amp or volt rating.

leej · Nov 21, 2013

(quoted from post at 23:15:29 11/21/13) So what does the grey stripe signify?

ts like this: the symbol on drawings is an arrow head pointing to a bar or line. On the part, that band corresponds to the line or bar. When those itty bitty current bits come down the line, if they come in a direction in which the arrow head is pointing, the point helps them punch through the bar/line/band and continue on their merry way, but.........if they try coming in the opposite direction, there is no pointy arrow head to help them punch thru the bar/line/band and they are stopped dead in their tracks. Got it?

Husker44A · Nov 21, 2013

That's a bandaid in my opinion. Yes it'll work fine but why not
fix it right by installing a new alternator with good diodes. Have
them test the new alternator diodes first before leaving the
store.

36 Coupe · Nov 21, 2013

You can buy a 1 amp 1000v diode for 10 cents,less by the hundred count.

David G · Nov 21, 2013

Diodes should NEVER be hooked in parallel. The one with the least resistance will take ALL the current.

Mike (WA) · Nov 21, 2013

I know about arrows and diodes. So the color of the stripe doesn't mean anything, other than what color of paint the Acme Diode and Aluminum Storm Door Co. happened to have in their gun when they made it?

old · Nov 21, 2013

I will make this extra easy for you. Radio Shack part #276-1661 is on that is more then enough to do the job and cost around $3 for a pack of four and all you need is one. Install it in the #1 excite wire with the band close to the alternator. I keep a pack of them on hand almost all the time for just that reason

Anonymous-0 · Nov 21, 2013

he's not talking about the diode trio he is asking about the diode in the small wire so that it doesn't backfeed and keep the engine running

JDemaris · Nov 21, 2013

Using an external diode has nothing to do with something being wrong with a Delco SI series alternator. A diode is used on the exciter circuit to prevent the alternator from back-feeding and preventing engine shut off. If a three-wire hook-up is used - even with a perfect alternator - a resistor or diode is one way to prevent back-feed.
Another way is to use an ignition switch with ACC and IGN terminals. Another way is to wire the excite circuit through an oil pressure switch. Another way it to just install a self-excite regulator and eliminate the exciter circuit.

JDemaris · Nov 21, 2013

I've seen SI series (10SI, 12SI and 18SI) draw up
to 2 amps @ 12 volts on the field/excite circuit.
So in reality - a single 3 amp diode is fine. Two
in parallel just gives extra insurance. So does
using a 5 amp or 10 amp diode. You can't use one
that rated for too much amps. But if you use one
for too little - it will blow like a fuse.

Many automotive circuits use a resistor instead of
a diode. It can be done either way - or with
neither and a switch used instead. Momentary,
pressure sensing, etc. The problem with a
resistor is - if not matched correctly -enough
current can pass and prevent engine shut-down.
This is especially true with diesels with electric
shut-offs. Unlike a resistor that just lowers
voltage (resists current and does not stop it) - a
diode is an electrical one-way check-valve.

John B. · Nov 21, 2013

We use to use diodes when I worked in St. Louis years ago to keep the alternator from back feeding the ignition switch when the engine would not shut off. They do sell a resistor that bolts on the back of the alternator for this. It takes the place of the idiot lite. You can also just hook the alternator wire up to the accessory terminal of your ignition switch. This will prevent feed back when you shut off your engine.

JDemaris · Nov 21, 2013

Yes, using the ACC terminal is the most trouble-free way to do it. Problem is - most tractors don't have one. In that case -it only works if you install a new ignition switch that has the isolated ACC and IGN terminals.

A resistor does not always work either. Especially on diesels with electric shut-offs that will operate on very low voltage.

John B. · Nov 21, 2013

I'll have to remember that.

mmidlam · Nov 22, 2013

I like to use an idiot light in parallel with 50 resistor instead of a diode. If the engine is not running and the key gets left on, the battery won't be drained near as fast compared to using a diode.
Engine might still start a few days later...no way if using the diode method.

cannonball · Nov 22, 2013

the first one I did back in the seventies, I used a push button...just have to remember to push it when you started it..

Can I use an 8 amp diode to stop alt leaking back on switch?

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